Visitors from other galaxies: stars that could reach the Milky Way from the outside

The Milky Way contains more than one hundred billion stars. Most are in a disc with a dense and bulky center, in the middle of which there is a supermassive black hole, while the rest is distributed in a much larger spherical halo. The stars spin around our galaxy at hundreds of kilometers per second, and their movements contain a large amount of information about their past. The fastest class of stars that can be found there is called hypervelocity stars, which are believed to begin their life near the galactic center and then launch themselves towards the edge of the Milky Way through interactions with the black hole.

In April, ESA’s top surveyor Gaia produced an unprecedented catalog of more than a billion stars, in which astronomers from around the world have worked over the past few months, scrutinizing the properties and movements of stars in our galaxy and beyond with precision never before reached. During this work, a team of astronomers using the latest data set of ESA’s Gaia mission to search for high-speed stars that were ejected from the Milky Way were surprised to find stars heading inward, perhaps from another galaxy.

The scientists, some of the University of Leiden (Holland), were surprised. Out of 1,300 million stars, Gaia measured the positions, the parallaxes, an indicator of their distance, and the 2D movements in the plane of the sky. In seven million of the brightest, he also measured how quickly they approach or move away from us. “Of the seven million Gaia stars with full 3D speed measurements, we found twenty that could travel fast enough to escape the Milky Way,” explains Elena Maria Rossi, one of the authors of the new study.

Rossi and his colleagues, who had already discovered a handful of hypervelocity stars last year in an exploratory study based on data from Gaia’s first launch, were pleasantly surprised, as they expected to find at most one star that escapes the Galaxy among these Seven million. But “instead of getting out of the galactic center, most of the high-speed stars we saw seem to be running towards it,” adds co-author Tommaso Marchetti, who says it could be “stars from another galaxy, moving through the Milky Way”.

The Large Magellanic Cloud, a possible origin

It is possible that these intergalactic intruders come from the Large Magellanic Cloud, a relatively small galaxy that orbits the Milky Way, or they can originate from an even farther galaxy.

If that is the case, they carry the trace of their home site and studying them at much closer distances than their mother galaxy could provide unprecedented information about the nature of the stars in another galaxy. “Stars can accelerate at high speeds when they interact with a supermassive black hole, so the presence of these stars could be a sign of such black holes in nearby galaxies, but stars may also have been part of a system. binary, launched into the Milky Way when its companion star exploded like a supernova, anyway, studying them could tell us more about this kind of processes in nearby galaxies, explains Rossi.

An alternative explanation is that the newly identified sprite stars could be native to the halo of our galaxy, accelerate and push inward through interactions with one of the dwarf galaxies that fell into the Milky Way during its accumulation history. Additional information on the age and composition of the stars could help astronomers clarify their origin. “A star in the halo of the Milky Way is likely to be quite old and made mostly of hydrogen, while the stars of other galaxies could contain many heavier elements.” Looking at the colors of the stars tells us more about what they are made of “, concludes Marchetti.